5 Answers
5

In order for your users to use your site the most efficiently, you must think of every situation possible that they might think of. Predicting what the user will want and delivering it to them before they are even there is good usability. Providing multiple locations for a perma-link allows for Stack Exchange to be easier to use, if you can get the perma-link from the address bar, the title, etc.

When looking at routes that users can take, you want to provide as many routes as possible to get there and / or get something accomplished, mainly because not everyone thinks the same and the idea to go straight to address bar to get the perma-link might come after going to the title of the page. The title of the page is bigger and easier to hit with the mouse.

Consistency:

If you click on the Questions link, it will still be there on the Questions page. The reason is because you want to form the best mental map in the users mind that you can. So, having links change creates uncertainty and confusion for the user. So, when you have the home button on the homepage, and the user goes to about and sees that the about link is still there, they know that the navigation will always be there to anchor on to.

That goes into the psychological analysis of web design... which can get kind of confusing all on it's own.

As a counterpoint to the other answers, this was actually discouraged at one point as potentially confusing to search engine bots if I recall correctly. It may still be, but I'm having some trouble digging up a citation at the moment(which may itself be an indicator this is no longer the case).

The idea is that Google and other web spiders use the words in the link text to get a better idea of what the target page is all about. This includes "self" links. Having a link with words actually related to the target pages contents would be better (from an SEO point of view) than using the word "permalink" (not much point in optimising for the search query "permalink").

It is sometimes hard to know how much emphasis to put on such advice given the difficulty in verifying the importance in actual relevance ranking on a site like Google. Such ideas can therefore persist for a long time since they can be neither proved nor disproved.